Last update: October 24, 2013 at 9:33 am by By Armand Vervaeck and James Daniell

Understanding the Bohol, Philippines earthquake

What happened exactly in Bohol?
At 8:12 AM on 15 October 2013, Tuesday, a destructive earthquake of magnitude 7.2 shook the island of Bohol and nearby provinces. Smaller-magnitude earthquakes followed afterwards, and as of 1:00 pm, 16 October 2013, 885 earthquakes have been recorded by the PHIVOLCS seismic monitoring network. At least 15 events were reportedly felt in the epicentral area. The main shock and succeeding aftershocks were located in the vicinity of Bohol. These recorded events were shallow, with a depth of at most 32 kilometers. Based on spatial distribution of succeeding events and characteristics of the earthquake, the event is tectonic in origin.
Based on preliminary intensity reports, the strongest ground shaking at PEIS VII (comparable with MMI VIII) was felt at Tagbilaran City and several cities in the province of Cebu. Neighboring island provinces of Cebu, Negros Occidental, Negros Oriental, Camiguin, Panay, Leyte, and several areas in northeastern Mindanao felt the earthquake at varying intensities of PEIS I-VI.

Moderate-magnitude (M5 to 6.9) earthquakes have also affected Bohol Island in the past!
On 08 February 1990, a magnitude 6.8 earthquake occurred at Bohol generated by an offshore reverse fault east of the island. Sixteen municipalities felt the strongest intensity of ground shaking at PEIS VIII. There were reports of severe property damages, numerous casualties, hundreds injured, and several thousands homeless. The towns of Jagna, Duero, Guindulman, Garcia Hernandez, and Valencia experienced tsunami inundation.

Why do earthquakes occur in Bohol?
Bohol Island is one of the seismically active areas in the country. Instrumental monitoring of earthquakes for the past century has detected many small to moderate-magnitude earthquakes in Bohol Island. There is at least one known earthquake generator on the island, the East Bohol Fault. In addition, there are other local faults which can be sources of small to large magnitude earthquakes. Earthquakes can also occur offshore or undersea because of local offshore faults near the island or trenches in the vicinity of the region.

Can these present earthquakes indicate volcanic activity?No. There are no volcanoes in Bohol Island.

What can we expect from the current earthquake activity?
The current seismic trend indicates that the magnitude 7.2 earthquake on 15 October 2013 is the mainshock, and the succeeding small magnitude earthquakes are the aftershocks. Aftershocks are expected, some of which will be felt (ER:and some of them might be dangerous). These may continue for weeks to months, but diminishing in number and strength as time passes. In this case, a higher magnitude earthquake related to this event is no longer expected to occur (ER: True, not related to this event, but as earthquakes cannot be predicted and as the Philippines have many fault systems, a new powerful earthquake can never be excluded).

What can we expect after a large-magnitude/high-intensity earthquake like this?
People are reminded to be cautious of structures visibly weakened or with signs of damage by the 15 October 2013 earthquake, as these may be further damaged by succeeding earthquakes. Strong ground shaking may cause extensive damage to or even the collapse of houses, buildings, bridges, and other infrastructures. Collapsed structures usually account for most of the casualties during a strong earthquake. Falling objects may also cause injuries.

Congratulations to Phivolcs Philippines for their very good explanation of this earthquake

Why are some places sinking and is the sea retreating away from the shore in other locations ?
Answer : deformation and partly liquefaction. In some locations
Isla Batasan is one of the islands located between Bohol and Cebu. It is experiencing sudden floods, causing its residents to believe that their island is slowly sinking. Phivolcs explains that there is indeed a big possibility that some islands may start sinking after the earthquake, due to liquefaction and earthquake deformation. Liquefaction happens when the soil loses its strength and stiffness due to an earthquake, causing it to soften and behave like liquid. Earthquake deformation, on the other hand, refers to a change in the original shape of a material.

Retreating sea : This can only be explained by an upthrust of some parts of the island. More detailed measurements in the near future will show serious deformations as can be seen on the images below. The focal mechanism below shows a mainly thrust earthquake = 2 parts of the fault are pressed into each other. The earthquake was triggered when the extreme forces became too big. The released energy did create all the deformations.

Retreating see in the thrust part of the island

Sinkholes are also generated by the shaking of the underground. Some sinkholes are produced when underground caves are caving in (the island of Bohol had a lot of caves) others are made by the shaking of loose material (some sources say that limestone is abundant in the islands).

Tectonic summary according to the USGS (a little more detailed)
The October 15, 2013 M 7.1 earthquake near the city of Catigbian on Bohol Island, Philippines, occurred as the result of shallow reverse faulting on a moderately inclined fault dipping either to the northwest, or to the southeast. The depth of the event indicates it ruptured a fault within the crust of the Sunda plate, rather than on the deeper subduction zone plate boundary interface. At the latitude of this earthquake, the Philippine Sea plate moves towards the west-northwest with respect to the Sunda plate at a rate of approximately 10 cm/yr, subducting beneath the Philippine Islands several hundred kilometers to the east of the October 15 earthquake at the Philippine Trench.
The Philippine Islands straddle a region of complex tectonics at the intersection of three major tectonic plates (the Philippine Sea, Sunda and Eurasia plates). As such, the islands are familiar with large and damaging earthquakes, and the region within 500 km of the October 15 earthquake has hosted 19 events of M6 or greater, a dozen of which have been shallow (0-70 km).

Update 23.10.2013 13:20 UTC:
Bad news once again with now 209 presumed dead. The other numbers remain the same.
The infrastructure cost has risen to 1.426 billion PHP counted (approx. 35 million USD).

Update 23.10.2013 04:35 UTC:
Unfortunately again we have had a rising death toll without reducing the missing. 195 dead and 12 missing is the current count.
– 651 have been injured
– 53,000 homes have been damaged – 14,000 of these have been destroyed.
– The public infrastructure loss including all hospitals, buildings, roads, flood control, schools is 1.097 billion PHP (25.47 million USD)
– 344,437 people are displaced (approximately the same amount of people as after the Tohoku earthquake.

Update 22.10.2013 07:35 UTC:
– All towns are now fully accessible by road given the hard work of DPWH in the last week. This is a huge accomplishment and will make it easier for relief to flow.
– The estimates of homeless range from 70,000 to 150,000 long term, with 377,000 currently displaced.
– The affected number has dropped below 3 million with a few barangays removing their “affected status”.

Update 22.10.2013 03:05 UTC:
The fault has been found! A previously unknown fault caused the Bohol earthquake. It caused at least 3m movement across a road in Inabanga. Click above to view the news article (GMA).
– The total number of destroyed buildings is at 14,253 (extra buildings in Loon and Maribojoc counted). It is still expected to rise further.
– The total number of damaged buildings is at 39,186.

for more information and to see others affected, photos, current conditions, etc., go to: http://earthquake-report.com/2013/10/15/very-strong-earthquake-mindanao-philippines-on-october-15-2013/